Biofuels. Sequential volarization of waste coffee grounds to Biodiesel, Bioethanol, and solid fuel

Table of Contents

List of Tables

List of Figures

List of equations

List of Appendices

Acronyms

1. INTRODUCTION
1.1. Background and justification
1.2. Problem of the statement
1.3. Objectives
1.3.1. General objective
1.3.2. Specific objectives
1.4. Significance of the study

2 . LITERATURE REVIEW
2.1. Overview of Ethiopia’s Energy Sector
2.2. Introduction to biodiesel
2.3. Biodiesel production
2.3.1. The Transesterification (alcoholysis) Process of Biodiesel
2.3.1.1. Catalytic transesterification
2.4. Variables Affecting the Transesterification Process
2.4.1. Effect of free fatty acids (FFA) and moisture
2.4.2. Effect of Alcohol/oil Molar Ratio and Alcohol Type
2.4.3. Type and Amount of Catalyst
2.4.4. Reaction Time and Temperature
2.5. Parameters which define the fuel quality of biodiesel
2.5.1. Density (15 οC)
2.5.2. Viscosity (40 οC)
2.5.3. Gross calorific value
2.5.4. Cloud point (CP)
2.5.5. Cetane Number
2.5.6. Iodine Value (IV)
2.5.7. Flash point
2.5.8. Water and sediment
2.5.9. Carbon Residue
2.5.10. Sulfated ash
2.5.11. Acid value
2.6. Coffee production and Waste coffee residues
2.6.1. Coffee production in Ethiopia
2.6.2. Waste Coffee Residues (WCRs)
2.6.2.1. Chemical composition of WCR
2.7. Advantages and Disadvantages of Biodiesel

3. MATERIALS AND METHODS
3.1. Materials and chemicals
3.2. Experimental
3.2.1. Waste coffee residue (WCR) Moisture content Determination
3.2.2. Waste coffee residue (WCR) oil extraction
3.2.3. Physicochemical parameters of WCRs oil
3.2.3.1. Determination of Saponification Value
3.2.3.2. Determination of Peroxide value
3.2.4. Two-Step Biodiesel Production Process
3.2.4.1. Acid-catalyzed esterification
3.2.4.2. Base-catalyzed transesterification
3.2.4.3. Purification
3.2.5. Biodiesel yield
3.2.6. Characterization of WCR Biodiesel
3.2.6.1. Determination of specific gravity/density (ASTM D1298) by hydrometer method
3.2.6.2. Determination of Kinematic Viscosity (40 οC)
3.2.6.3. Determination of Acid value
3.2.6.4. Determination of Gross calorific value
3.2.6.5. Determination of Cloud point (ASTM D 2500)
3.2.6.6. Determination of Water and sediment
3.2.6.7. Determination of Cetane number
3.2.6.8. Determination of Iodine Value
3.2.6.9. Determination of flash point by Pensky-Martens closed cup tester
3.2.6.10. Determination of conradson carbon residue
3.2.6.11. Determination of Ash content
3.2.6.12. Determination of Copper strip corrosion
3.2.6.13. Determination of distillation characteristics
3.2.7. Fatty acid composition of WCR methyl ester
3.2.8. Production of Bioethanol from the solid waste remaining after oil extraction of WCR (Spent of WCR)
3.2.9. Determination of the quality of the solid residue after Bioethanol production for compost and solid fuel
3.2.9.1. Calorific value
3.2.9.2. Proximate analysis

4. RESULTS AND DISCUSSION
4.1. Oil content of waste coffee residue (WCR)
4.2. Physicochemical characteristics of the extracted WCR oil
4.3. Biodiesel yield of WCR oil
4.4. Waste Coffee Residue methyl ester Fuel properties
4.4.1. Density (15 οC)
4.4.2. Kinematic viscosity (40 οC)
4.4.3. Gross calorific value
4.4.4. Cloud point
4.4.5. Acid value (AV)
4.4.6. Cetane number
4.4.7. Iodine Value
4.4.8. Water and sediment
4.4.9. Distillation Temperature
4.4.10. Ash content
4.4.11. The Conradson Carbon residue of waste coffee residue ester
4.4.12. Flash point
4.4.13. Copper corrosion
4.5. Fatty acid composition of WCR Biodiesel
4.6. Bioethanol yield from solid waste remaining after oil extraction of WCR (Spent of WCR)
4.7. Solid fuel and compost from the WCR after Bioethanol production
4.7.1. Solid waste remaining after Bioethanol production for solid fuel
4.7.2. Solid waste remaining after Bioethanol production for compost

5. CONCLUSION AND RECOMMENDATIONS
5.1. Conclusion
5.2. Recommendations

REFERENCES

APPENDICES

List of Tables

Table 2.1: Sector wise energy source utilization percentage distribution

Table 2.2: Composition of waste coffee residue

Table 3.1: Standard test methods for physicochemical properties of WCR oil

Table 3.2: test methods to characterize WCR oil methyl ester/biodiesel

Table 3.3-Analysis of solid fuel and compost

Table 4.1 - Moisture and oil contents of WCR

Table 4.2: Characterization of the oil extracted from waste coffee Residue

Table 4.3: Characterization of the WCR methyl ester

Table 4.6: characteristics of WCR after bioethanol production for solid fuel and compost

List of Figures

Figure 1: Scheme of general transesterification reaction

Figure 2: A typical base-catalyzed process for the production of bio-diesel

Figure 3: A typical acid-catalyzed process for the production of bio-diesel

Figure 4: Experimental set up for Soxhlet Extraction

Figure 5: Rotavapor used to separate hexane from the extracted WCR oil

Figure 6: WCRs before extraction (left) and after extraction (right)

Figure 7: General Flow chart of the study

Figure 8: Transesterification of WCR oil

Figure 9: During phase separation Purification stage Purified WCR biodiesel

Figure 10: Cannon- Fenske glass capillary viscometer tube with samples in the SETA KV-8 viscometer bath

Figure 11: The Peltier device apparatus

Figure 12: Centrifuge for determination of water and sediment

Figure 13: Pensky- Martens closed cup tester

Figure 14: Carbon residue test apparatus assembly at pre ignition stage

Figure 15: The samples in crucible at ignition stage

Figure 16: Copper Corrosion Apparatus

Figure 17: Setastill distillation apparatus

Figure 18: a. Hydrolysis b. Fermentation c. fractional distillation d. FTIR reading

Figure 19: WCR oil

Figure 20: Density of WCR ester in relative to biodiesel EN 14214 standard specification

Figure 21: WCR ester viscosity in comparison with ASTM and EN biodiesel standards

Figure 22: Acid value of WCR oil and ester in relation with ASTM and EN standards

Figure 23: cetane number of WCR oil and ester in relation with ASTM and EN standards

Figure 24: Iodine value of WCR oil and ester in relation with EN standard specification

Figure 25: Distillation temperature of WCR biodiesel in relative to ASTM specification

Figure 26: WCR ester ash content in comparative to ASTM and EN specifications

Figure 27: WCR ester flash point in relation to ASTM and EN standard specifications

Figure 28: WCR methyl ester fatty acid composition

Figure 29: Bioethanol yield (%) of the solid waste after oil extraction (spent of WCR)

List of equations

Equation 1: Mosture determination

Equation 2: Waste coffee residue oil content determination

Equation 3: Determination of saponification value

Equation 4: Determination of peroxide value

Equation 5: Acid pretreatment loss

Equation 6: Determination of biodiesel yield (%)

Equation 7: Kinematic Viscosity Determination

Equation 8: Acid Value Determination

Equation 9: Determination of calorific value

Equation 10: Cetane index determination

Equation 11: Cetane number determination

Equation 12: Determination of iodine Value

Equation 13: Corrected flash point

Equation 14: Carbon residue determination

Equation 15: Ash determination

Equation 16: Determination of Boiling tempratuer

Equation 17: Gram of bioethanol

Equation 18: Yield of bioethanol

Equation 19: Fixed carbon determination

List of Appendices

Appendix 1: ASTM D6751-09 Standard Specification for Biodiesel Fuel (B100)

Appendix 2: EN 14214 biodiesel fuel standard

Appendix 3: Fatty acid composition of waste coffee residue oil methyl ester

Appendix 4: Gas chromatogram profile of fatty acid methyl ester of WCR

Appendix 5: Distillation Characteristics of WCR Methyl Ester

Appendix 6: Bioethanol yield from spent of WCR

Appendix 7: absorbance reading of spent of WCR bioethanol with distilled water and 1 molar H2SO

Appendix 8: Proximate analysis of the waste solid after bioethanol production87 Appendix 9: calculations for esterification and transesterification

Appendix 9.1: calculation for Esterification

Appendix 9.2: calculation for Transesterification

Acronyms

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1. INTRODUCTION

1.1. Background and justification

In recent times, the world has confronted with crises of increased demand for energy, price hike of crude oil, global warming due to emission of green house gases, environmental pollution, and fast diminishing supply of fossil fuels (Atadashi, et al., 2011; Miguel and Calixto, 2009). The indiscriminate exploration and consumption of fossil fuels has led to a reduction in petroleum reserves (Miguel and Calixto, 2009).

Our reliance on these energy sources threatens energy security and influence economic growth especially in fuel importing countries like Ethiopia. About all of Ethiopia’s liquid fuel requirements are imported in the form of refined petroleum products (Alemayhu, 2007). This external energy supply is unsteady and has become a burden to the rapidly growing national economy. In addition, diesel powered motor vehicles in the road transport sector are an important contributor to the total gas emissions in the urban cities (Christoffel, 2010). From the point of view of global environment protection and the concern for long-term supplies of conventional diesel fuels, it becomes necessary to develop alternative fuels comparable with conventional fuels. Alternative fuels should be, not only sustainable but also environmentally friendly (Miguel and Calixto, 2009). Some of the most notable alternative sources of energy capable of replacing fuels (Miguel and Calixto, 2009) include amongst others: water, solar and wind energy, and biofuels (Atadashi et al., 2011). A potential diesel oil substitute is biodiesel (Miguel and Calixto, 2009).

Biodiesel is a new energy source that has grown in importance over recent years. Nowadays, used vegetable oils are potential renewable sources for the production of biodiesel as an alternative to petroleum based diesel fuel, which is derived from diminishing petroleum reserves and which has environmental consequences caused by the exhaust gases from diesel engines (Maceiras et al., 2009). Biodiesel has several benefits such as a diminution in greenhouse gas emissions: it reduces emissions of carbon monoxide by about 50% and emissions of carbon dioxide by about 78% (Sheehan et al., 2008). In addition, biodiesel is produced from a variety of vegetable oils (such as soybean, rapeseed and sunflower) and animal fats, and can be used in diesel engines blended with petroleum diesel or on its own (Sánchez et al., 2012). Researchers are developing certain crops with high oil content just for the production of biodiesel (Azam et al., 2005; Cardone et al., 2003; Encinar et al., 2002; Gressel, 2008) or looking for new sources to produce biodiesel (Kondamudi et al., 2008). Therefore, it will be very useful to look for new raw sustainable materials for biodiesel production that do not involve the use of cereals and plants that compete with land.

Coffee is one of the largest agricultural products that are mainly used for beverages (Kondamudi et al., 2008) throughout the world and providing approximately 30.6% of Ethiopia's foreign exchange earnings in 2010-2011 (Bureau of African Affairs, 2012). Ethiopia is currently producing an estimated 9.804 million 60-kg bags that would rank as the third largest coffee producer in the world after Brazil and Vietnam (African Commodity Report, 2012) and half of the coffee is consumed by Ethiopians (Abu, 2012).

Waste coffee residue (WCR), the solid dregs found from the treatment of coffee powder with hot water to prepare instant coffee, is the main coffee industry residues with a generation of 6 million tons worldwide (Tokimoto et al., 2005) and 235,296 tons in Ethiopia. According to Kondamudi et al. (2008), on a worldwide scale, based on the amount of coffee that is used, 340 million gallons of biodiesel can be produced from Waste coffee residues. Simões (2009) demonstrated that WCR can be used for the production of biodiesel and fuel pellets and as a source of polysaccharide with immune stimulatory activity. The amount of oil in the Waste coffee residue source varies from 11 to 20 wt % depending on its types (Arabica and Robusta) (Daglia et al., 2008), which is roughly equivalent to that of palm, rapeseed, and soybean sources (Kondamudi et al., 2008). Compared to other waste sources; such as cooking oils, animal fats, and other biomass residues, coffee has the additional benefits of being less expensive, more stable due to the presence of antioxidants, and comprising of a nice smell (Al-Hamamre et al., 2012). On average, the Waste coffee residues comprise 15% oil, by weight, which can be converted to a similar amount of biodiesel using transesterification methods (Kondamudi et al., 2008).

The biodiesel from waste coffee residues possesses better stability than biodiesel from other sources due to its high antioxidant content (which hinders the rancimat process) (Campo et al., 2007; Yanagimoto et al., 2004). WCR is also considered an inexpensive and easily available adsorbent for the removal of cationic dyes in wastewater treatments (Franca et al., 2009). However, none of these strategies have yet been routinely implemented, and most of these residues remain unutilized, being discharged to the environment where they cause severe contamination and environmental pollution problems due to the toxic nature (presence of caffeine, tannins, and polyphenols) (Leifa et al., 2000). Nowadays, there is great political and social pressure to reduce the pollution arising from industrial activities. In this sense, conversion of WCR to value-added compounds is of environmental and economical interest (Mussatto et al., 2011). In this work, investigation of WCR as a potential raw material for biodiesel production and its by-products for bioethanol and solid fuel as well as compost was carried out.

1.2. Problem of the statement

In today’s world, alternative fuels are needed more than ever. The primary sources of energy are mainly non-renewable: natural gas, oil, coal, peat, and conventional nuclear power. These Conventional fuels are constantly being depleted; however, our dependency on these fuels is still growing. Additionally, the price on foreign fuels is ever increasing. For these reasons, Ethiopia is pursuing alternative fuel sources to lessen the dependency on conventional fuel that is petro. One alternative fuel is biodiesel; biodiesel can be produced from vegetable oil or animal fat and thus can be used to alleviate the foreign fuel dependency. In order for biodiesel to be a viable alternative fuel source, a cheap feedstock for biodiesel production process needs to be improved. Compared to current designs and fossil fuel, the process must be cost competitive.

Ethiopia imports its almost all petroleum fuel requirement and the demand for petroleum fuel is rising rapidly due to a growing economy and expanding infrastructure (Alemayhu, 2007). Statistics from the Ethiopian Ministry of Mines and Energy (MoME) indicate that the country spends about Ethiopian Birr 10 billion (US$800 million) annually to import petroleum products for domestic consumption. This astounding figure represents nearly 90 percent of the earnings that the country makes each year in foreign trade. By cutting its dependency on foreign oil, Ethiopia could perhaps keep some of the money inside the country (Gathanju, 2010).

1.3. Objectives

1.3.1. General objective

· To Investigate the Waste Coffee Residue (WCR) remaining after brewing coffee as a potential alternative raw material for biodiesel production.

1.3.2. Specific objectives

- To extract oil from WCR remaining after brewing coffee
- To analyze Physicochemical parameters of WCR oil that affect the production and characteristics of the biodiesel produced from the oil
- To produce biodiesel from WCR and to analyze its fuel properties such as acid value, density, kinematic viscosity, iodine value, flash point, cetane number, carbon residue, water and sediment content, heating value and cloud point, along with the standard test methods
- To determine fatty acid composition of the biodiesel
- To evaluate the spent remaining after the oil extraction for possible uses as fuel and compost

1.4. Significance of the study

The results of this study will give insight to produce biodiesel from waste materials. Clean energy for today's economic development is crucial to assure a sustainable development. Hence, the investigation of waste coffee residue remaining after brewing coffee as a potential alternative raw material for biodiesel production is very timely because of arising problems such as the rising cost of fuel in the market, global warming phenomenon, and health problems such as respiratory diseases caused by the harmful byproducts of burning petroleum-based fuels. Producing a biodiesel from it will be a good way to minimize GHG and waste in the environment.

2. LITERATURE REVIEW

2.1. Overview of Ethiopia’s Energy Sector

Ethiopia is well endowed with a variety of energy and other natural resources. However, much of the energy resource available has yet to be exploited. The renewable energy resources with potential include biomass, hydropower, and alternative forms of energy-solar, wind and geothermal energy. There are also considerable reserves of coal and natural gas (W/Giorgis, 2004).

Most people living in Ethiopia have, until now, been unable to satisfy their household energy requirements with modern energy sources (kerosene, electricity, gas). In rural areas they use only biomass (wood, dung or agricultural waste) for cooking, baking and heating. The biomass energy consumption is estimated about 94% of the total. The low agricultural production is a consequence of deforestation, erosion and desertification. On the other hand petroleum products which are used mainly at urban household center are entirely an imported commodity. Demand for these products is rising rapidly increasing due to in scarcity of fuel wood and the change of life style of the people. The rise in demand is accompanied by a much faster growth in the import bill because of rising petroleum prices and products (Alemayhu, 2007).

In Ethiopia the gross available potential land for production of feedstock for biodiesel is estimated about 23,305,890 hectares and the total irrigable land for sugarcane production for ethanol production is about 700,000 hectares. Thus Ethiopia has a potential to produce 1billion liters of ethanol within available suitable land (Alemayhu, 2007). The major use of energy, about 89% of the overall energy consumption in the country, is the households. The second most important sector in terms of energy consumption is industry (4.5%) followed by services and others (3.6%) while agriculture and transport were attributed to the remaining 2.3%. The consumption of energy is directly related to the availability of energy source, the size of the population and the price (Ministry of mines and energy, 2011).Table 2.1 shows the sector-wise percentage usage distribution of energy source type in Ethiopia.

Table 2.1: Sector wise energy source utilization percentage distribution

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Source: (Meskir, 2007; Ministry of mines and energy, 2011)

2.2. Introduction to biodiesel

Biodiesel is defined as “a substitute for, or an additive to diesel fuel that is derived from the oils and fats of plants and animals” (Ma and Hanna, 1999) or a fuel composed of monoalkyl esters of long-chain fatty acids (Abreu et al., 2004; Hass et al., 2001) produced by transesterification reaction of vegetable oils and animal fats with short chain alcohols meeting the requirements of ASTM D6751 (ASTM, 2008; Burtis,2006; Encinar et al., 2002; Noureddini et al.,1998). The alcohol and the base compound (lye) are used to split vegetable oil into components. The ester component is what is known as biodiesel, while the separated by-product is glycerin (Alberta, 2006). Glycerin has value as a co-product and can be used in soaps, lotions, and as a lubricant (Sawyer, 2007). Biodiesel is a form of biofuel that is a nontoxic, biodegradable substance and does not contain any sulfur (Alberta, 2006). It is an alternative to diesel fuel produced from domestic renewable resources. Typically biodiesel derived from the oils and fats of that of sunflowers, soybean, canola, and rapeseeds, can be used in diesel engines with no to little modifications (Liu et al., 2010).

Similar to diesel, biodiesel can function as an alternative to electricity and petroleum diesel. Because its components are derived from renewable resource and has a lower emission compared to the traditional petroleum fuel, biodiesel is environment-friendly. With every one unit of needed energy to generate biodiesel, in turn, 4.5 units of energy is return. This is due to biodiesel having a high-energy balance and is locally produced (National Biodiesel Board, 2010).

2.3. Biodiesel production

Bio-diesel production is a very modern and technological area for researchers due to the relevance that it is winning everyday because of the increase in the petroleum price and the environmental advantages (Marchetti et al., 2005). The most common method of producing bio-diesel is the reaction of vegetable oils and animal fats with alcohol mainly methanol in the presence of catalyst. This process is called transesterification and is not a new process. It was conducted as early as 1853 by two scientists E. Duffy and J. Patrick. Since that time several studies have been carried out using different oils such as soybean (Watanabe et al., 2002), rapeseed (Kusdiana and Saka, 2001), cotton seed (Royon et al.2007), waste cooking (Zhang et al., 2003), spent coffee ground (Daglia el al., 2008; Kondamudi et al., 2008),sunflower seed (Harrington and D’Arcy-Evans,1985) , winter rape (Peterson et al., 1991), different alcohols such as methanol (Demirbas, 2006), ethanol (Encinar et al., 2002).

2.3.1. The Transesterification (alcoholysis) Process of Biodiesel

The vegetable oils and animal fats usually contain free fatty acids, phospholipids, sterols, high viscosity, water, odourants and other impurities. Because of these, direct use of these vegetables and animal fats oil as fuel for diesel engine can cause particle agglomeration, injector fouling due to its low volatility and high viscosity, which is about 10 to 20 times greater than petroleum diesel (Fan, 2008) . To reduce these problems the oil requires chemical modification principally through transesterification, pyrolysis and emulsification (Ma and Hanna, 1999). Among these, the transesterification is the main and fore most important step to produce the cleaner and environmentally safe fuel from vegetable oils (Knothe and van Gerpen, 2005; Meher et al., 2004). Additionally, the physical properties of biodiesel produced by this simple process are very close to the petroleum diesel fuel (Fan, 2008).

Transesterification is the general term used to describe the important class of organic reactions where an ester is transformed into another through interchange of the alkoxy moiety (Freedman et al., 1986). It is the reaction of vegetable oil or animal fat with an alcohol, in most cases methanol, to form esters and glycerol. According to Srivastava & Prasad (2000) transesterification is the displacement of alcohol from an ester by another alcohol in a process similar to hydrolysis, except that alcohol is employed instead of water. The transesterification reaction is affected by alcohol type, molar ratio of glycerides to alcohol, type and amount of catalyst, reaction temperature, reaction time and free fatty acids and water content of vegetable oils or animal fats. The transesterification process consists of a sequence of three consecutive reversible reactions, which include conversion of triglycerides to diglycerides, followed by the conversion of diglycerides to monoglycerides. The glycerides are converted into glycerol and yield one ester molecule in each step.

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Fig. 1: Scheme of general transesterification reaction (source: Ma and Hanna, 1999).

2.3.1.1. Catalytic transesterification

Catalytic transesterification is the process by which different catalysts are used to initiate the esterification for making biodiesel. Also known as methanolysis, this process is well studied and established (Helwani et al. 2009). The three basic Catalysts of biodiesel production from oils/fats are the base-catalyzed transesterification, the acid catalyzed esterification, and enzymatic catalysis (Kaieda et al., 1999; Haas et al., 2006; Ma and Hanna, 1999; Meher et al., 2006). Among these base-catalyzed transesterification is involved in biodiesel production today (Srivastava and Prasad, 2000; Zhang et al., 2003), where feedstocks with a high water or free fatty acid (FFA) content needs pretreatment with an acidic catalyst in order to esterify FFA (Freedman et al., 1984; Kaieda et al., 1999).This is the most common method done because it is the most economical (Singh et al, 2006).

2.3.1.1.1. Base-Catalyzed transesterification

Base-catalyzed transesterification involves stripping the glycerin from the fatty acids with a catalyst such as sodium or potassium hydroxide and replacing it with an anhydrous alcohol, usually methanol. The resulting raw product is then centrifuged and washed with water to cleanse it of impurities. This yields methyl or ethyl ester (biodiesel), as well as a smaller amount of glycerol, a valuable by-product used in making soaps, cosmetics and numerous other products (Refaat, 2010).The base-catalyzed transesterification of vegetable oils and fats to form alkyl ester is faster than the acid-catalyzed reaction. It is the one most used technique commercially as it is the most economical process since it requires only low temperatures and pressures; produces over 98 % conversion and involves direct conversion to biodiesel with no intermediate compounds; also, no special materials of construction are needed (Dube et al., 2007; Freedman et al., 1984; Singh et al., 2006). The most commonly used alkali catalysts are NaOH, CH3ONa, and KOH (Korytkowska et al., 2001; Varghaa et al., 2005; Vicente et al., 2004). Alkyl oxide solutions of sodium methoxide or potassium methoxide in methanol, which are now commercially available, are the preferred catalysts for large continuous-flow production processes (Singh et al., 2006).

Base-catalyzed transesterification, however, has some limitations among which are removal of these catalysts is technically difficult and brings extra cost to the final product (Demirbas, 2003; Goff et al., 2004) and it is sensitive to FFA content of the feedstock oils and water (Canakci et al., 2003; Furuta et al., 2004; Leung and Guo, 2006). The starting material should have free acid content less than 0.5% (acid value less than 1) and water content less than 0.3% (Wright et al., 1944). Other disadvantage of the base-catalyzed transesterification is that the process is energy intensive, recovery of glycerol is difficult, alkaline catalyst has to be removed from the product and alkaline waste water requires treatment (Meher et al., 2006). A typical base-catalyzed process for the production of bio-diesel is shown in Fig. 2.

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Fig. 2: A typical base-catalyzed process for the production of bio-diesel (source: Mohammed, 2011).

2.3.1.1.2. Acid-Catalyzed Transesterification

Acid-catalyzed transesterification is slower than base-catalyzed transesterification. The reaction temperature is usually above 100 °C and reaction time is 3 to 48 hours except when the reaction is conducted under high temperature and pressure (Allen et al. 1945; Taylor et al. 1927). The procedure of acid-catalyzed transesterification is different from base-catalyzed one. The reaction is refluxed at or near the boiling point of the mixture of cosolvent and alcohol. The transesterification process is catalyzed by sulfuric (Goff et al., 2004; Liu et al., 2006; Lopez et al., 2005), hydrochloric (Lee et al., 2000; Goff et al., 2004), and organic sulfonic acids (Stern and Hillion, 1990). A simplified block flow diagram of the acid-catalyzed process is shown in Fig. 3. In general, acid catalyzed reactions are performed at high alcohol-to-oil molar ratios, low-to moderate temperatures and pressures, and high acid catalyst concentrations. However, ester yields do not proportionally increase with molar ratio. For instance, for soybean methanolysis using sulfuric acid, ester formation sharply improved from 77% using a methanol-to-oil ratio of 3.3:1 to 87.8% with a ratio of 6:1. Higher molar ratios showed only moderate improvement until reaching a maximum value at a 30:1 ratio (98.4%) (Lotero et al., 2006). Despite its insensitivity to free fatty acids in the feedstock, acid-catalyzed transesterification has been largely ignored mainly because of its relatively slower reaction rate (Zhang et al., 2003).

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Fig. 3: A typical acid-catalyzed process for the production of bio-diesel (source; Mohammed, 2011).

2.3.1.1.3. Enzyme-Catalyzed Transesterification

Enzyme-Catalyzed Transesterification is gaining more attention nowadays and has the potential to do better than chemical catalysts for biodiesel production in the future. New biochemical routes to biodiesel production, based on the use of enzymes, have become very interesting (Chang et al., 2005; De Oliveira et al., 2004; Noureddini et al., 2005). Most of the articles published have used a variety of substrates such as rice bran oil, canola, sunflower oil, soybean oil, olive oil, and castor oil. Several lipases from microbial strains, including Candida Antarctica (Lai et al., 2005; Royon et al., 2007), Candida rugasa (Chen and Wu, 2003; Linko et al., 1998), Pseudomonas cepacia (Deng et al., 2005; Shah and Gupta; 2007), Thermomyces lanuginosus (Xu et al.,2004), Pseudomonas sp. (Lai et al., 1999), and Rhizomucor miehei (Lai et al., 1999; Skagerlind et al.,1995) have been reported to have transesterification activity. The enzymatic alcoholysis of soybean oil with methanol and ethanol was investigated using a commercial, immobilized lipase (Bernardes et al., 2007; De et al., 2006). In that study, the best conditions were obtained in a solvent-free system with ethanol/oil molar ratio of 3.0, temperature of 50 °C, and enzyme concentration of 7.0% (w/ w). They obtained yield 60% after 1 h of reaction.

The advantages of lipase-catalyzed transesterification, compared to the chemically-catalyzed reaction, are emphasized and can be reused without separation (Nelson et al., 1996; Shimada et al., 2002). Also, the operating temperature of the process is low (50 °C) compared to other techniques (Nelson et al., 1996; Shimada et al., 2002). The main problem of the lipase-catalyzed process is the high cost of the lipases used as catalyst (Royon et al., 2007) and inhibition effects which were observed when methanol was used (Nelson et al., 1996; Shimada et al., 2002).

2.4. Variables Affecting the Transesterification Process

There are number of factors which could affect the transesterification process. These factors include moisture content, free fatty acid contents, molar ratio of oil to alcohol, type and amount of catalyst, reaction time, reaction temperature, mixing intensity, and co-solvent (Demirbas, 2007; Ma & Hanna, 1999; Meher et al., 2006; Sharma et al., 2008). These factors or variables usually have different effect on the transesterification process depending on the method used for the transesterification process. The effects of these factors are described below.

2.4.1. Effect of free fatty acids (FFA) and moisture

The free fatty acids (FFA) and moisture contents are two key parameters for determining the viability of the feedstock (vegetable oil) transesterification process (Meher et al., 2006). In the transesterification, FFAs and water always produce negative effects, since the presence of FFAs and water causes soap formation, consumes catalyst and reduces catalyst effectiveness, all of which result in a low conversion (Demirbas, 2006 and 2007). Generally, moisture and free fatty acids content of the feedstock must be reduced or lowered (<0.5% for homogenous process) to avoid its undesirable effect on the catalyst and the transesterification reaction (Ma & Hanna, 1999; Sharma et al., 2008). The higher the acidity of the oil, smaller is the conversion efficiency. Both, excess as well as insufficient amount of catalyst may cause soap formation (Meher et al., 2004) . In homogenous transesterification process especially, moisture and free fatty acids in the feedstock could lead to a side saponification reaction, which produces soap and eventually emulsion (Bkansedo, 2009). The resulting soap and emulsion can induce an increase in viscosity, formation of gels and foams, and make the separation and purification process of the final product, which eventually leads to loss of triglyceride and product (biodiesel)(Ghadge and Raheman; 2005). The presence of water has a greater negative effect on transesterification than that of the FFAs. Ma et al. (1998) investigated the transesterification of beef tallow catalyzed by sodium hydroxide (NaOH) in presence of FFAs and water. These authors reported that water and free fatty acid contents of beef tallow had to be maintained below 0.06 wt% and 0.5 wt%, respectively. Using NaOH catalyzed transesterification, methyl esters can generally be prepared in high yields for low FFA oils, being nearly quantitative for the palm oils containing <1% FFA. For example, the yield of methyl esters from RBD (refined, bleached and deodorized) palm oil with about 0.05% FFA was 98% (May,2004).

2.4.2. Effect of Alcohol/oil Molar Ratio and Alcohol Type

One of the most important factors that affect the yield of ester is the molar ratio of alcohol to triglyceride. Although the stoichiometric molar ratio of methanol to triglyceride for transesterification is 3:1, higher molar ratios are used to enhance the solubility and to increase the contact between the triglyceride and alcohol molecules (Ma & Hanna, 1999; Noureddini et al., 1998). Higher molar ratios result in greater ester conversion in a shorter time. Hence, in the transesterification process more alcohol is preferred to shift the equilibrium to form higher yield of fatty acid alkyl esters. Molar ratio of alcohol to oil at 6:1 is considered as the standard ratio (Fukuda et al., 2001; Gerpen, 2005). However, other researches also shown that molar ratio of alcohol to oil from 5:1 (Alamu et al., 2008) up to 8:1 (Ramadhas et al., 2005), 9:1 (Sahoo et al., 2007), 12:1 (Meher et al., 2006), and higher could also be used as the optimum ratio for oil to methanol, depending on the quality of feedstock and method of the transesterification process.

According to Meher et al. (2006) the reaction was faster with higher molar ratio of methanol to oil whereas longer time was required for lower molar ratio (6:1) to get the same conversion. In their research, the molar ratio of methanol to oil, i.e., 6:1, 9:1, 12:1, and 24:1, were examining for optimizing biodiesel production from Karanja oil. However, when the ratio of oil to alcohol is too high, it could give adverse effect on the yield of fatty acid alkyl esters. Some researchers reported that addition of large quantity of methanol, i.e. at ratio of 1:70 and 1:84 could slow down the separation of esters and glycerol phases during the transesterification process, therefore affecting the final yield of fatty acid alkyl esters (Miao & Wu, 2006).

2.4.3. Type and Amount of Catalyst

The type and amount of catalyst required in the transesterification process usually depend on the quality of the feedstock and method applied for the transesterification process. For a purified feedstock, any type of catalyst could be used for the transesterification process (Gerpen, 2005). Catalysts used for the transesterification of triglycerides are classified as alkali, acid,enzyme or heterogeneous catalysts, among which alkali catalysts like sodium hydroxide, sodium methoxide, potassium hydroxide and potassium methoxide are more effective, much faster than acid-catalyzed transesterification and is most often used commercially (Ma & Hanna, 1999). May (2004) studied the effect of different catalysts types on methanolysis of palm oil with a low FFA content of <0.1%. In that study, it was concluded that NaOH and KOH are effective catalysts. Stavarache et al. (2005) investigated the effect of different catalyst concentrations on base-catalyzed transesterification during bio-diesel production from vegetable oil by means of ultrasonic energy. The best yields were obtained when the catalyst was used in small concentration, i.e., 0.5% wt/wt of oil. Meneghetti et al. (2006) investigated the effect of different catalyst types at different temperatures during the production of free and bound ethyl ester (FAEE) from castor oil. Results from that study showed that hydrochloric acid is much more effective than sodium hydroxide at higher reaction temperatures.

The yield of fatty acid alkyl esters generally increases with increasing amount of catalyst (Demirbas, 2007; Fukuda et al., 2001; Ma & Hanna, 1999). This is due to availability of more active sites by additions of larger amount of catalyst in the transesterification process. However, on economic perspective, larger amount of catalyst may not be profitable due to cost of the catalyst itself.

2.4.4. Reaction Time and Temperature

Freedman et al (1986) observed the increase in fatty acid esters conversion when there is an increase in reaction time. The reaction is slow at the beginning due to mixing and dispersion of alcohol and oil. After that the reaction proceeds very fast. However the maximum ester conversion was achieved within < 90 min. Further increase in reaction time does not increase the yield product i.e. biodiesel/mono alkyl ester (Leung and Guo, 2006; Alamu et al., 2007). Ma et al. (1998) evaluated the effect of reaction time on transesterification of beef tallow with methanol. Due to the difficulty of mixing and dispersion of methanol into beef tallow, the reaction was very slow during the first minute. From 1 to 5 min, the reaction proceeds very fast. At about 15 min, the production of beef tallow methyl esters reached the maximum value. Current researches have shown that the reaction time for a non-catalytic transesterification process using supercritical alcohol is shorter compared to conventional catalytic transesterification process (Demirbas, 2003 and 2005).

Reaction temperature is another important factor that will affect the yield of biodiesel.Transesterification can occur at different temperatures, depending on the oil used. For the transesterification of refined oil with methanol (6:1) and 1% NaOH, the reaction was studied with three different temperatures (Freedman et al., 1984). After 0.1 h, ester yields were 94, 87 and 64% for 60, 45 and 32 °C, respectively. After 1 h, ester formation was identical for 60 and 45 °C runs and only slightly lower for the 32 °C run. Temperature clearly influenced the reaction rate and yield of esters (Ma & Hanna, 1999). The boiling point of methanol is 337.8 K. Reaction temperatures higher than this will burn the alcohol and will cause reduced yield. Leung & Guo (2006) indicated that reaction temperature higher than 323 K had a negative impact on the product for neat oil.

2.5. Parameters which define the fuel quality of biodiesel

The characteristics of biodiesel are similar to those of diesel fuel, and, therefore, biodiesel becomes a strong candidate to replace the diesel fuel if the need arises. The fuel properties of biodiesel are influenced at large by the amounts of each fatty acid composition and the alcohol moieties in the feedstock used to produce the esters among which the largest fractions of fatty acids for each of the biodiesel is a potential indication of the rest of the properties (Kinast, 2003; Van Gerpen et al., 2004). The important physical and chemical properties of oil and biodiesel which determined by standard methods were used as follow:

2.5.1. Density (15 °C)

Density is specified in several standards and the purpose is to exclude unrelated materials from being used as biodiesel feedstock (Knothe, 2006). It is also used in the determination of the viscosity of biodiesel. This property is important mainly in airless combustion systems because it influences the efficiency of atomization of the fuel (Felizardo et al., 2006). Density, relative density or specific gravity is a factor governing the quality and pricing of biodiesel. However, this is uncertain indication of its quality unless correlated with other properties. Density was measured with a hydrometer in accordance with ASTM D1298, Standard Test Method for Density, Relative Density (Specific Gravity) (ASTM Standard D1298, 2005).

Biodiesel is generally denser than diesel fuel with sample values ranging from 877 kg/m3 (tallow methyl ester) to 884 kg/m3 (soy methyl ester) compared with diesel at 835 kg/m (Sharp, 2000).Thus; density of the final product depends mostly on the feedstock used. The transesterification process has been found in some cases to reduce fuel density (Zheng and Hanna, 1995). Lower density contaminants such as methanol and ethanol also decrease overall density of the fuel.

2.5.2. Viscosity (40 °C)

Viscosity is a measure of the internal fluid friction or resistance of oil to flow, which tends to oppose any dynamic change in the fluid motion (Sivaramakrishnan and Ravikumar, 2012) and is determined by measuring the amount of time taken for a given measure of oil to pass through an orifice of a specified size (Allen et al, 1999). It is one of the parameters specified in biodiesel and petro-diesel standards that require compliance since it affects the operation of fuel injection equipment, particularly at low temperatures when the increase in viscosity affects the fluidity of the fuel (Canoira et al., 2009; Knothe, 2005). Biodiesel has higher viscosity than conventional diesel fuel. High viscosity leads to poorer atomization of the fuel spray and less accurate operation of the fuel injectors (Strong et al., 2004). Knothe and Steidley (2005), reported that the presence of an OH group in ricinoleic acid increases viscosity significantly oil due to hydrogen bonding. Therefore the major reason for reducing viscosity of processing plant oils is to make them suitable for use as biodiesel. Methods of reducing viscosity in addition to transesterification include dilution, microemulsion, pyrolysis and catalytic cracking (Canakci and Gerpen, 2001; Demirbas, 2006; Gemma et al., 2004; Hirata and Berchmans, 2007).

Ranges of acceptable kinetic viscosity at 40 °C are 1.9 – 6.0 mm2/s as required by ASTM D6751 specification. The kinetic viscosity of fatty compounds (such as those found in biodiesel fuel) is significantly influenced by compound structure, including chain length, the position, number, and nature of double bonds, and the nature of oxygenated moieties (Knothe et al., 2005). Biodiesel viscosity is also dependent on temperature. It is reported that biodiesel viscosity can be calculated in the range from 273 K to 303 K with one equation (Kerschbaum and Rinke, 2004).

2.5.3. Gross calorific value

Calorific value of a fuel is the thermal energy released per unit quantity of fuel when the fuel is burned completely and the products of combustion are cooled back to the initial temperature of the combustible mixture. It measures the energy content in a fuel. This is an important property of the bio-diesel that determines the suitability of the material as alternative to diesel fuels (Sivaramakrishnan and Ravikumar, 2012). The heat content of vegetable oils and their alkyl esters is nearly 90% that of DF and the heats of combustion of fatty esters and triacylglycerols are in the range of ~1300– 3500 kg cal/mol for C8– C22 fatty acids and esters (Freedman and Bagby, 1989). One of the most important determinants of heating value is moisture content. The calorific value of vegetable oils and their methyl esters were measured in a bomb calorimeter according to ASTMD240 standard method (Sivaramakrishnan and Ravikumar, 2012; Parr, 1987)). Benzoic acid was used to standardize the calorimeter. One gram of sample was taken in a crucible and made into a pellet and the initial weight was noted. It was placed in the bomb, which is pressurized to 18 atm of oxygen (Mesfin, 2008). For purposes of comparison, the literature value for the heat of combustion of cetane is 2559.1 kg-cal/mol (at 20 °C); thus it is the same range as fatty compounds (Weast et al, 1985–1986).

2.5.4. Cloud point (CP)

Low temperature operability of biodiesel fuel is an important aspect from the engine performance standpoint in cold weather conditions (ASTM Standard D2500, 2005). There are several tests that are commonly used to determine the low temperature operability of biodiesel. CP is one of these tests and is included as a standard in ASTM D6751. It is the temperature at which crystals of organic matter in the biodiesel are visualized when measured by lowering its temperature according to the procedures described by ASTM D2500. ASTM D6751 requires the producer to report the cloud point of the biodiesel sold, but it does not set a range as the desired cloud point is determined by the intended use of the fuel (ASTM Standard D2500, 2005).

While operating an engine at temperatures below oil’s cloud point, heating will be necessary in order to avoid waxing of the fuel. Small crystals of fuel begin to form in the liquid causing haziness as the sample is cooled. It is an indicator of the utility of petroleum oil for some applications. In the case of biodiesel, the haze is made up of crystallized fuel molecules, specifically crystallized stearic and/or palmitic methyl esters (Knothe and Dunn, 2001).

2.5.5. Cetane Number

Cetane Number (CN) or aniline point is a relative measure (the scale) of the interval between the beginning of injection and auto-ignition of the fuel (conceptually similar to the octane scale used for gasoline). The CN is the primary specification measurement used to match fuels and engines (Van Gerpen et al., 2004). In a compression ignition diesel engine the cetane number is the measure of ignition promotion. In a spark ignited gasoline engine the ignition quality of gasoline is measured by the octane number which is a rating of ignition delay (Midwest Biofuels, 1994). The cetane number is not to be confused with the cetane index, which is not applicable to biodiesel. Cetane indices predict the cetane number from equations derived for petroleum distillates only. The cetane number of biodiesel depends on the distribution of fatty acids in the original oil or fat from which it was produced. The longer the fatty acid carbon chains and the more saturated the molecules, the higher the cetane number (Van Gerpen et al., 1996).

The cetane number of a fuel reflects its ignition delay. A fuel of higher cetane number gives lower delay period and provides smoother engine operation (Mittelbach and Gangl, 2001). Therefore, high cetane number is desirable for engine fuel. Biodiesel has a higher CN than petrodiesel because of its higher oxygen content (Mittelbach and Gangl, 2001). Cetane numbers of biodiesels differ depending on the respective oil sources. Cetane numbers of biodiesel from soybean oil methyl esters lie between 45.8 and 56.9, and that from rapeseed oil methyl esters lie between 48 and 61.8. Cetane increases with chain length, decreases with the number of double bonds, and decreases as double bonds and carbonyl groups move towards the centre of the chain. Increasing cetane number of biodiesel has been shown to reduce nitrogen oxides emissions (Van Gerpen et al., 2004).

2.5.6. Iodine Value (IV)

Iodine value (number) is a measure of the total unsaturation within a mixture of fatty acids, and is expressed in grams of iodine which react with 100 grams of biodiesel. Engine manufacturers have argued that fuels with higher iodine number tend to polymerize and form deposits on injector nozzles, piston rings and piston ring grooves when heated (Kosmehl and Heinrich, 1997). Moreover, unsaturated esters introduced into the engine oil are suspected of forming high-molecular compounds which negatively affect the lubricating quality, resulting in engine damage (Schaefer et al., 1997). Biodiesel viscosity is directly correlated to the iodine number of biodiesel for biodiesel with iodine numbers of between 107 and 150 (Prankl and Worgetter, 1996).

2.5.7. Flash point

The flash point is a measure of the lowest temperature at which application of the flame causes the vapor above the sample to ignite, i.e., it is a measure of the tendency of a sample to form a flammable mixture with air (Van Gerpen et al., 2004).It is the lowest temperature at which fuel emits enough vapors to ignite (ASTM Standard D93, 2008). FP varies inversely with the fuel’s volatility (Sivaramakrishnan and Ravikumar, 2012). It specifies the temperature to which a fuel needs to be heated for the vapour and air above the fuel could be ignited (Sarma, 2005). Minimum flash point temperatures are required for proper safety and handling of diesel fuel. Fire point is the lowest temperature at which a specimen will sustain burning for 5 seconds (Sivaramakrishnan and Ravikumar, 2012). Biodiesel has a high flash point; usually more than 150°C, while conventional diesel fuel has a flash point of 55-66°C (Knothe et al., 2005). Furthermore, the flash point of methyl ester fuels is higher than that of ethyl esters. If methanol, with its flash point of 12°C is present in the biodiesel the flash point can be lowered considerably (Mallinckrodt Baker Inc., 2009). Flash points of the samples were measured in the temperature range of 60 to 190°C by an automated Pensky-Martens closed cup apparatus (Sivaramakrishnan and Ravikumar, 2012).

2.5.8. Water and sediment

Water and sediment is a test that determines the volume of free water and sediment in middle distillate fuels having viscosities at 40 °C in the range 1.0 to 4.1 mm2/s and densities in the range of 700 to 900 kg/m3 (Van Gerpen et al., 2004).

This test is a measure of cleanliness of the fuel. For B100 it is particularly important because water can react with the esters, making free fatty acids, and can support microbial growth in storage tanks. Water is usually kept out of the production process by removing it from the feedstocks. However, some water may be formed during the process by the reaction of the sodium or potassium hydroxide catalyst with alcohol. If free fatty acids are present, water will be formed when they react to either biodiesel or soap. Finally, water is deliberately added during the washing process to remove contaminants from the biodiesel. This washing process should be followed by a drying process to ensure the final product will meet ASTM D 2709 (Van Gerpen et al., 2004).Sediments may plug fuel filters and may contribute to the formation of deposits on fuel injectors and other engine damage. Sediment levels in biodiesel may increase over time as the fuel degrades during extended storage (Van Gerpen et al., 2004).

2.5.9. Carbon Residue

An important indicator of the quality of biodiesel is the carbon residue, which corresponds strictly to the content of glycerides, free fatty acids, soaps, remaining catalyst and other impurities (Mittelbach, 1996).This parameter indicates the tendency of the fuel to form carbon deposits in an engine. Carbon residue which is formed by decomposition and subsequent pyrolysis of the fuel components can clog the fuel injectors. ASTM D6751 includes carbon residue as a standard for biodiesel. The maximum allowable carbon residue for biodiesel is 0.050 % by mass (ASTM Standard D6751, 2009).

“In petroleum products, the part remaining after a sample has been subjected to thermal decomposition...” is the carbon residue. The carbon residue is a measure of how much residual carbon remains after combustion. The test basically involves heating the fuel to a high temperature in the absence of oxygen. Most of the fuel will vaporize and be driven off, but a portion may decompose and pyrolyze to hard carbonaceous deposits. This is particularly important in diesel engines because of the possibility of carbon residues clogging the fuel injectors (Van Gerpen et al., 2004).

2.5.10. Sulfated ash

Sulfated ash is the residue remaining after a fuel sample has been carbonized, and the residue subsequently treated with sulfuric acid and heated to a constant weight. This test monitors the mineral ash residual when a fuel is burned (Van Gerpen et al., 2004). Ash is formed from abrasive solids, soluble metallic soaps and unremoved catalysts remaining in the biodiesel. Combustion in the engine oxidizes these materials to ash. The ash content is specified in standards either as ash (oxidized) content or sulfated ash content. There is a correlation between the sulphated ash content and the phosphorous content of the oil (Mittelbach, 1996). For biodiesel, this test is an important indicator of the quantity of residual metals in the fuel that came from the catalyst used in the esterification process. Producers that use a base catalyzed process may wish to run this test regularly. Many of these spent sodium or potassium salts have low melting temperatures and may cause engine damage in combustion chambers (Van Gerpen et al., 2004).

2.5.11. Acid value

Acid value or neutralization number is a measure of free fatty acids contained in a fresh fuel sample and of free fatty acids and acids from degradation in aged samples. If mineral acids are used in the production process, their presence as acids in the finished fuels is also measured with the acid number. It is expressed in mg KOH required to neutralize 1g of biodiesel. It is influenced on the one hand by the type of feedstock used for fuel production and its degree of refinement. Acidity can on the other hand be generated during the production process. The parameter characterizes the degree of fuel ageing during storage, as it increases gradually due to degradation of biodiesel. High fuel acidity has been discussed in the context of corrosion and the formation of deposits within the engine which is why it is limited in the biodiesel specifications of the three regions. It has been shown that free fatty acids as weak carboxylic acids pose far lower risks than strong mineral acids (Cvengros, 1998). Acidity can also result from improper manufacturing, through remaining catalyst (if manufactured in acidic conditions) or excessive neutralization. This is different for diesel, which does not contain these materials (Scherpenzeel, 2000).

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